I've bent over many nails in a salt water environment, some of them I've been able to observe for as long as 40 years. I've never noticed an increased corrosion rate, except when I damaged the zinc coating.

Oxidation occurs most easily at points of stress in a metal. This point becomes an anode for the reaction of Iron with Oxygen, yielding the half-reaction of Fe→Fe2++2e-. The reduction half-reaction, therefore, is O2+2H2O+4e-→4OH-.

There are some nice illustrations of this on pages 816-817 of Chemistry, 9th Edition by Whitten, Davis, Peck, & Stanley.

The reason for this increased oxidation at the points of stress is that the atomic structure allows the process to occur more readily. The section on Cold Worked Corrosion at http://www.efunda.com/materials/corrosion/corrosion_types.cfm has some minimal information on this, and you can always look it up in more depth if you're interested/this is the information that you need.